An optical recording medium for optically recording information allows high density recording, and shows excellent mobility, and the information recorded therein is not easily deteriorated, therefore, it has been increasingly demanded as the recent information society is further advanced. Such an optical recording medium includes, for example, an optical card, an optical disc such as a CD, a CD-ROM, and a CD-R. Recording and reproducing of the information is carried out using near infrared laser beams and the like.
This optical recording medium typically comprises a transparent protective layer and a substrate, and an optical recording layer held between them. For this transparent protective layer, a polycarbonate resin of a low price, having low water absorption property, which is resistant to warping or cracking and which shows an excellent bend property, has been often used. However, polycarbonate resins have a problem of low scratch resistance. Therefore, when a polycarbonate resin is employed for a transparent protective layer, a cured layer formed from an ion radiation curing type resin having a big curing shrinkage rate has been generally further provided in the past. With this cured layer, the surface of the optical recording medium can be made as hard as, or harder than pencil hardness (defined by JIS-K-5400) H which represents sufficient scratch resistance. In this way, it is intended that the optical recording medium is protected from scratches when it is carried with, or in use, and the durability of the optical recording medium as well as the reliability of the recording and reproducing of the information are improved.
Recently, there has been an increased demand to record an individual information after such an optical recording medium is produced. Examples of the recording of such individual information include, for example, a signature of the card holder, display of the card use status, and recording of various prints and the like, formation of a magnetic recording layer, and formation of a hologram and the like.
In the conventional optical recording medium, the material forming the cured layer for the surface thereof has excellent scratch resistance but inferior adhesion with a printing ink and the like. Therefore design printing, functional printing or magnetic layer formation has been difficult to be carried out on the cured layer. The use of a leveling agent such as silicone for leveling a resin layer of the cured layer deteriorates post-adhesion with the ink. The use of an ionized radiation curable resin having a little curing shrinkage rate can improve adhesion with a printing ink and the like. However, the resulting hardness can only be around F-HB in terms of pencil hardness, and the resulting cured layer has an insufficient scratch resistance disadvantageously. In addition, such a cured layer has poor adhesion with a magnetic recording layer or a hologram even an adhesive agent is employed, and there is the fear that it might come off or be peeled off during use.
The present invention has been developed in view of the above-mentioned problems, and an object of the present invention is to provide an optical recording medium, which allows post-processing such as printing on the cured layer which has been difficult to be carried out so far, without degrading the reliability of the information of the optical recording medium.